Why is the Leaning Tower of Pisa useful?

Answer 1

It's useful as a tourist attraction.

The probably-apocryphal story is that Galileo used the Tower of Pisa to demonstrate the uniform acceleration of Earth's gravity. He dropped two balls of the same size, but different weights, and observed that they hit the ground at the same time. This demonstrates that, neglecting air friction, gravity exerts the same accelerating force on all objects regardless of mass, i.e. heavier objects do not fall faster.

Of course, he did not need a tower with an offset base to test this theory.